<p>Excessive intermixing of the materials and thick intermetallic compounds (IMCs) could markedly impair the mechanical properties of Al-Cu dissimilar joints. In this study, a dynamic-stationary shoulder friction stir lap welding (DSSFSLW) was applied to Al-Cu lap welding, thereby reducing material mixing and the thickness of IMCs. Compared with traditional friction stir lap welding (FSLW), the shear strengths of DSSFSLW joints were higher than those of FSLW in the same welding parameters. The maximum shear strength could reach to 3.34 kN, which was 41.5% higher than that of FSLW joints. Failure was initiated on the Al side, immediately beside the SZ, and manifested as a cross-shaped crack. Furthermore, the formation mechanism of IMCs on the Al-Cu interface was elucidated. Quantitative analysis revealed that the thickness of IMCs on the side of SZ in the DSSFSLW joint was only 4μm, which was significantly lower than that of FSLW. Meanwhile, thermodynamic and kinetic studies indicated that the IMCs layer could be changed from Al₂Cu-AlCu-Al₄Cu₉ in the FSLW to Al₂Cu-Al₄Cu₉ by DSSFSLW. This study provides new insights for high-strength of Al-Cu lap joints.</p>